132 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



and the increasing weights temporarily increase the irritability and therefore 

 increase the amount of shortening. The effect of tension to increase the activ- 

 ity of the contraction process is seen if a muscle which is connected with a 

 strong spring or heavy weight be excited to isometric contractions and in 

 the midst of a contraction be suddenly released ; the muscle under such cir- 

 cumstances is found to contract higher than when excited by the same stimulus 

 without being subjected to tension. The effect of tension on the activity of 

 muscular contractions is to be clearly seen in the case of the heart muscle. 

 A rise of pressure of the fluid within the isolated heart of a frog increases 

 the strength as well as the rate of the beat. 



If the weight be gradually increased, although the height of the contrac- 

 tions is lessened, the work will for a time increase, and a curve of work (con- 

 structed by raising ordinates of a length corresponding to the work done, 

 from points on an abscissa at distances proportional to the weights em- 

 ployed), will be seen to rise. After the weight has been increased to a cer- 

 tain amount the decline in the height of contractions will be so great that the 

 product of the weight into the height will begin to decrease, and the curve of 

 work will fall, until finally a weight will be reached which the contracting 

 muscle can just support at, but not raise above, its normal resting length. As 

 has been said, this weight will be a measure of the absolute muscular force. 



Example. 



Load Height of lift Work 



(grams). (millimeters). (grammillimeters). 



13 



30 11 330 



60 9 540 



90 7 630 



120 5 600 



150 3 450 



180 2 360 



210 



In the above experiment 30 grams was added to the muscle after each 

 contraction; as the weight was increased up to 90 grams the amount of 

 work was increased, with greater weights the amount of work was lessened. 



Liberation of Thermal Energy. Energy leaves the body as mechanical 

 energy only when by its movements the body imparts energy to surrounding 

 objects. Most of the energy liberated within the body leaves it as heat ; even 

 during violent muscular exercise five times more energy may be expended as 

 heat than as mechanical energy, and the disproportion may be even greater 

 than this. So great is the production of heat during exercise, that, in spite 

 of the great amount leaving the body, the temperature of an oarsman has been 

 found to be increased, during a race of 2000 meters, from 37.5 C. to 39 

 or 40 C. 1 



It is exceedingly difficult to ascertain with accuracy on the warm-blooded 

 animal the exact relation of heat-production to muscular contraction. The 

 1 Geo. Kolb: Physiology of Sport, translated from the German, 2d edition, London, 1892. 



